Digital optical data storage media of the WORM type (Write Once, Read Many times) stores information written on a thin layer by means of a light pulse of high intensity, typically a strongly focused laser beam. Once data is written on to the layer, the layer cannot be returned to its original state, but it can be read many times by a weaker light beam which does not further influence the layer's physical state. Well known means in this context are thin metal film, flat glass or plastic surfaces or thin polymer film which contains a light-absorbing dye. In most cases such layers are provided between other layers, e.g. reflecting or protective layers. The layer stores information by undergoing a change under the influence of the powerful write light pulse and constitutes the actual data storage layer in the medium.
For the most part today's optical storage technology is almost exclusively based on the reflective contrast between written marks in the data storage layer or its immediate surroundings. A focused laser beam is passed along the data storage layer and changes in the intensity of the reflected laser light are recorded when the beam passes a write mark. A typical mark may be in the form of small round or elongated pits, with dimensions of between 0.5 and 7 .mu.m. Optical data storage media which are based on reflection normally employ a strongly reflecting layer, e.g. a vaporized aluminium layer in a multilayer structure where the data storage layer regulates the amount of light incident on the medium which is reflected.
Data storage media are also known which are based on transmission contrast for light which is detected after having passed through the data storage medium. In this case, the data storage layer can have a low light transmission capacity in an unknown condition and be more transparent at the write marks which are formed by the powerful write beam. Alternatively the data storage layer can be converted from transparent to opaque at the write marks.
It is also known that data storage media are under development in which the contrast is based on stimulated light emission. During the reading of data one or more light beams scan the data storage medium which reacts by emitting light of an intensity which is dependent on how the medium was treated early in the write phase. The light energy emitted results from either the release of captured electrons in a high energy state in the data storage layer or down conversion of the scanning light beam. Holographic data storage media have also been proposed, with storage of data both on volume and layers. An important contrast mechanism in this connection is light-induced alteration of the refraction index.
Data storage media may also be based on the fluorescent properties, of a data storage layer. For instance, EP-A1-503428 discloses an erasable optical recording medium comprising at least one recording layer which includes a fluorescent material and a photo-reactive bistable quencher capable of quenching the fluorescence emitted from the fluorescent material. The photo-reactive bistable quencher produces two isomers, one having an absorption wavelength spectrum which is different from the other and the one being convertible into the other by an irradiation with light. One of these bistable isomers acts as a quencher for quenching the fluorescence emitted by the fluorescent material. Particularly the photo-reactive bistable quencher is a photo-chromic compound having a quenching action. Information is digitally recorded by utilising the two bistable isomers of the photo-reactive bistable quencher. In the erased state the absorption spectrum of the photo-bistable quencher is that of the long wavelength isomer, while in the recorded state the absorption spectrum is that of its short wavelength isomer. The information is recorded by causing the fluorescent material to absorb the irradiated light to transfer the absorbed energy to the photo-reactive bistable quencher, causing the photo-reactive quencher to participate in an isomerization reaction from the long wavelength isomer to the short wavelength isomer. The reading of the recorded information is performed either by detecting the fluorescence emitted from the fluorescent material or by transforming only a small portion of the photo-reactive bistable quencher which is included in the recorded layer in an excessive amount from its short wavelength isomer back to its long wavelength isomer state, thus activating the quencher. It will be seen that according to EP-A1-503482 the quenching action takes place by a photo induced chemical reaction and entails no depletion of the fluorescent material in a recorded spot on the recorded medium.
Most of today's commercially available, optical data storage media, however, are based on data storage layers of the WORM type, based on one of the following categories:
1) Hole formation in an absorbing or reflecting thin film, where the hole is produced by ablation or melting of a metal film. Such data storage layers can often be used without an underlying reflector and provide contrast when light which passes through the hole after the hole formation is lost in the internal data storage layer or behind it, thus enabling a dark mark to be recorded on a strongly reflecting background which it is read under reflection. Data storage media of this kind are also well suited to be read by means of transmission, provided that relatively little light is able to pass through the data storage layer before it is written on it. PA1 2) Topographical mark formation by mass transport is known, e.g. in the form of a heat-induced pitting in a polymer film. During writing, a strong pulse is absorbed by focused light in the polymer film, causing local heating and transport of the polymer material away from the heated area. The physical process which is involved in the mass transport is normally thermoplastic deformation, possibly also evaporation or ablation in some cases. The result is that a pit is formed whose dimensions and shape are defined by the focal size of the light beam and the absorption efficiency of the polymer film, the duration of the exposure and the thermal diffusion and material transport parameters for the material in the write area in the data storage layer. Data are read from the medium by recording the total reflection coefficient for incident light which passes through the polymer layer and which is reflected from an underlying reflection layer and returns via the polymer film. In order to obtain the desired light-absorbing properties, the polymer film is usually treated with a dye. PA1 3) Other data storage media of a similar nature are also known, where the data storage layer's absorbing or reflecting properties are modified by embedding particles suitable for this purpose in a transparent base material or where the surface of the data storage layer is influenced locally by a thermal process induced by a high-powered, focused laser beam, thus causing the surface in the illuminated point to become smoother and more reflective.